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Developing components

Local development

Vite Sandbox

Running the Vite server for local development will spin up the component sandbox. Each component should have a simple working demo in the sandbox for local development and testing. These basic demos will also be used for testing components within MediaWiki.

To start the local dev server, run this command in the root of the Codex repository:

npm run dev

This will serve the sandbox page at http://localhost:5173, and automatically update it in the browser as you make changes to the code (hot module reloading). If you need it to be served on a different port, you can use npm run dev --port=12345.

To add a new component demo to the sandbox:

  • In packages/codex/src/demo, add a new .vue file containing the new demo
  • Import the new file into packages/codex/src/demo/Sandbox.vue, add it to the template, and add it to the demoSections array.

VitePress docs site

Aside from Vite Sandbox, you can also run the VitePress site locally to write and test a full suite of component demos. See the component demos section for more information.

Component basics

The codex package uses Vue 3 and prefers the Composition API over the Options API.

Codex is written in TypeScript: see the Working with TypeScript section for information about code conventions, solved problems, and potential pitfalls.


  • Export all components that are ready for public consumption in src/lib.ts to make them available to library users.
  • PascalCase multi-word component names are used per the Vue.js Style Guide. Component names should be prefixed with "Cdx," e.g. CdxButton
  • Slot names should be all lowercase. Use kebab-case for slot names with multiple words. This is necessary to ensure support for environments that use DOM templates, including MediaWiki.

WIP components

Until a component is ready for public consumption, it is considered a "work in progress" or "WIP" component. WIP components are housed in a separate directory, packages/codex/src/components-wip, and are included in index.ts in that same directory. WIP components will not be included in public dist files, nor will they appear on the docs site when viewing the documentation for a specific Codex release. However, WIP components will appear on the docs site for the main branch, and when serving the docs site locally.

Adding a WIP component

To add a new WIP component:

  • Add a new directory and .vue file to the WIP components directory, e.g. packages/codex/src/components-wip/my-new-component/MyNewComponent.vue
  • Import this component into packages/codex/src/components-wip/index.ts

Before submitting a patch for the new component, we recommend at least adding a basic demo to the sandbox and snapshot tests.

When adding examples for the VitePress demo page in the codex-docs package, the component should be imported from the @wikimedia/codex package as usual. This will not work in real code as long as the component is still WIP, but the codex-docs package is set up to be able to access WIP components this way.

Transition to public component

Once the entire design spec has been implemented, the component has a full suite of interactive demos, and the component has reached the unit test threshold, it is ready for public use. To make this transition, submit a patch that:

  • Moves the component directory from components-wip to components
  • Exports the component in lib.ts instead of components-wip/index.ts
  • Updates the paths used to import component, from ../../components/foo/Foo.vue to ../foo/Foo.vue.

Once this patch is merged, the component will be included in the dist and will appear on the docs site for the next release.

Writing styles

Styles are written in Less and are included in the single-file component at the end in a <style lang="less"> wrapped section. Codex design tokens are imported as Less variables from the @wikimedia/codex-design-tokens package, using the default theme-wikimedia-ui theme.


Design tokens

See tokens organization, naming and structure for a detailed overview.
Stylesheet specific token application rules:

  • If a component uses a value not represented by a Codex token yet, add a component-level Less variable in the <style> tag before the first selector.
  • Tokens should follow naming patterns established for MediaWiki.
  • Codex does not use shorthand properties background, font, animation and transition for simpler design token scoping and code modularization reasons. Only tokens of a category type are summarized into a shorthand token, e.g.
    "text-decoration": {
      	"none": {
      		"value": "{ text-decoration.0 }"
      	"line-through": {
      		"value": "{ text-decoration.150 }"
      	"underline": {
      		"value": "{ text-decoration.200 }"

Selectors and structure

  • A light version of BEM is used for class naming structure:
    • The root element will have the class .cdx-component-name
    • A block within that root element would have the class .cdx-component-name__block-name
    • A variation of that block would have the class .cdx-component-name__block-name--modifier-name
    • Codex actually expands the block logic structure for direct component children by a class like .cdx-component-name__block-name__block-child-name. While following this, there is no need to go deeper than 2 block levels in a class name; class names of further child elements can omit some of the blocks for the sake of brevity.
  • Add specificity when styling a sub-component to avoid override styles. For example, an Icon component that is inside a Label component would use the selector, .cdx-label__icon.cdx-icon. In Less:
    cdx-label {
    	&__icon.cdx-icon {
    		// ...rules...
  • If a style or selector isn't self-explanatory, add a comment above it in Less comment style //.
  • Avoid HTML element selectors. The style rulesets should aim to be independent from specific element choices, which may change.
  • Use --has- and --is- prefixes for modifiers that are not tied to a specific state, e.g. --has-thumbnail or --is-link.
  • Styles specific to a component's enabled or disabled state should be contained in a selector specific to that state. This structure allows us to avoid overriding enabled styles for the disabled state.
    • The pseudo-classes &:enabled and &:disabled can be used when available, otherwise &--enabled and &--disabled classes should be added (e.g. .cdx-menu-item--enabled). These are available to contain styles for the two states and for simpler styles orientation.
    • The stylelint no-descending-specificity rule can be disabled to maintain this structure (see sample code below).

Less mixin parameters

  • In order to distinguish clearer from CSS variables and Codex tokens, Less mixin function parameters should be prefixed with param (e.g. @param-size).


  • Codex uses stylelint-order to order CSS/Less rules
  • Enforce relying only on specific CSS properties over shorthands for background, font, animation and transition.
  • Stylelint rules can be disabled/enabled and should be marked as CSS style comment /* stylelint-disable-next-line rule-name */.

Below are some sample styles for a component to demonstrate these conventions:

<style lang="less">
@import ( reference ) '@wikimedia/codex-design-tokens/theme-wikimedia-ui.less';
@import ( reference ) '../../themes/mixins/binary-input.less';

.cdx-radio {
	// Common binary input styles.

	// Custom-styled radio that's visible to the user.
	&__icon {
		border-radius: @border-radius-circle;

		// …

	// HTML `<input type="radio">`.
	// Based on the HTML attributes of the radio input, we can change the style of the adjacent
	// `span`, which will look like a custom-styled radio.
	&__input {
		&:enabled {
			& + .cdx-radio__icon {
				border-color: @border-color-input-binary;

			// Note: there is no focus behavior for the input in its unchecked state because you
			// can't focus on it without selecting it.
			&:hover + .cdx-radio__icon {
				border-color: @border-color-input-binary--hover;

			// …

		/* stylelint-disable no-descending-specificity */
		&:disabled {
			// Only disabled radios should have a gray label.
			& ~ .cdx-radio__label {
				color: @color-disabled;

			// …
		/* stylelint-enable no-descending-specificity */

Bidirectional script support

Codex has limited support for bidirectional text. It supports pages that are entirely in a left-to-right (LTR) script, or pages that are entirely in a right-to-left (RTL) script. It does not support pages with a mix of LTR and RTL content, or pages whose directionality changes at runtime, except in some special cases. At the time of this writing, it's virtually impossible to support those use cases without the :dir() CSS pseudo-class, which is not yet supported by most browsers.

There are tools (like postcss-rtlcss, see below) that generate bidirectional CSS using attribute selectors like [dir='ltr'], but this technique is fragile. It breaks in confusing and ugly ways on pages that don't have a dir attribute set, and on pages where a dir="ltr" element is nested inside a dir="rtl" element or vice versa. Because of these significant limitations, bidirectional stylesheets are only useful in very limited circumstances, and Codex does not provide one.

Flipping of direction-specific styles

Styles in Codex are written for left-to-right (LTR) environments. Codex uses RTLCSS to generate flipped versions of these styles for right-to-left (RTL) environments. For example, a rule like .foo { padding-left: 4px; } will be changed to .foo { padding-right: 4px; } in RTL. In the build, the LTR styles are placed in and the RTL styles

In most cases, this automatic transformation should produce the correct result, but you should always test style changes in both LTR and RTL. Both the sandbox (npm run dev) and the component demos (npm run doc:dev) have direction switchers that allow you to switch between LTR and RTL on the fly.

In some cases, the automatic flipping transformation doesn't produce the correct result. Override directives can be used to address this. To prevent a rule from being flipped, put /* rtl:ignore */ on the line above it. To set a different value for a property in RTL, put the RTL value in a comment like /* rtl:4px */. These two directives are the most important ones, but others exist; see the postcss-rtlcss documentation on control directives and value directives for more information.

Below is an example that demonstrates these directives:

.foo {
	// This rule isn't flipped. It uses float: left; in both LTR and RTL
	/* rtl:ignore */
	float: left;

	// This rule is flipped, because there is no rtl:ignore directive above it
	// It becomes padding-right: 12px; in RTL
	padding-left: 12px;

.bar {
	// This rule uses -100% (`@size-full`) in RTL, because that value is explicitly specified.
	transform: translateX( 0 ) /* rtl:translateX( -@size-full ) */;

	// This rule is NOT flipped to margin-right, because an explicit RTL value is specified
	// In RTL, this rule becomes margin-left: 30px; (NOT margin-right: 30px;)
	margin-left: 12px /* rtl:30px */;

Direction-specific behavior in components

Some components need to adjust their behavior depending on the text direction. For example, components that listen for the left and right arrow keys being pressed may need to react to those key presses differently depending on the text direction.

To achieve this, components should use the useComputedDirection() composable, which detects the direction of the surrounding context of the component at mount time. This works even on pages with mixed or nested directionality; however it does not detect changes in directionality that happen after the component is mounted.

Below is an example that demonstrates the use of the useComputedDirection() composable function:

	<!-- Set ref="rootElement" on the root element of your component -->
		<!-- ...component template goes here... -->

import { defineComponent, ref } from 'vue';
import useComputedDirection from '../../composables/useComputedDirection';
// ...other imports...

export default defineComponent( {
	setup() {
		const rootElement = ref(); // If using TypeScript, use ref<HTMLDivElement>();
		const computedDir = useComputedDirection( rootElement );

		function onKeydown( e ) {
			if ( e.key === 'ArrowLeft' ) {
				// Left means "next" in RTL, "previous" in LTR
				navigate( computedDir.value === 'rtl' ? 'next' : 'previous' );
			} else if ( e.key === 'ArrowRight' ) {
				navigate( computedDir.value === 'ltr' ? 'previous' : 'next' );

		function navigate( prevOrNext ) {
			// ...

		return {
} );

The Icon component also uses this composable to detect the text direction, and allows the detected direction to be overridden through the dir prop. For more information about how bidirectionality is handled for icons in particular, see the icon documentation.

Inheriting attributes

By default, components will place any attributes bound to them on the root element of the component. Sometimes, though, we don't want this behavior. For example, for a component that contains an <input> element, we may want to bind most of the attributes to that <input> element rather than the root element.

Some attributes, however, should always be bound to the root element in order to provide expected results for library users. This includes class and style attributes.


Binding a style attribute to a component is highly discouraged as it could interfere with Design System consistency and negatively impact performance. Nonetheless, if one is provided, it will be bound to the root element of the component.

To help achieve the desired behavior in components like this, we have a composable called useSplitAttributes. It provides the following:

  1. rootClasses: all CSS classes that should be bound to the root element, including those set via the class attribute on the component and those that are internal to the component, like dynamic and conditional classes
  2. rootStyle: the style attribute bound to the component, should one be provided
  3. otherAttrs: all other attributes, which can be bound to the desired child element

Below is sample usage from the TextInput component:

	<!-- Add rootClasses and rootStyle to the root element. -->
		<!-- Bind otherAttrs to the input. -->

// Import the composable.
import useSplitAttributes from '../../composables/useSplitAttributes';

export default defineComponent( {
	name: 'CdxTextInput',
	// Set inheritAttrs to false.
	inheritAttrs: false,
	setup( props, context ) {
		// Define dynamic classes internal to the component, in Vue's object
		// syntax format.
		const internalClasses = computed( () => {
			return {
				'cdx-text-input--has-start-icon': !!props.startIcon,
				'cdx-text-input--has-end-icon': !!props.endIcon || props.clearable,
				'cdx-text-input--clearable': isClearable.value
		} );

		// Get helpers from the composable.
		const {
		} = useSplitAttributes( context.attrs, internalClasses );
} );